Know your audience. That's the first step in identifying what is fun or not. Even then it isn't 100% guaranteed though. Anyhow, how are the physics like? What's the age group? Art style? Mobile or different platform? iOS or android users? Are we talking about the animaniacs crowd for instance? Or are we making a game for Cthulhu fans?
So then, how could we make the game fun, with the physics. Borderlands art style with cartoonish explosions dropping from the sky blowing everything up, but with the right ragdoll physics it explodes and makes the bits and pieces from one creature kill the ones next to it. And then with a sound score that's some punk rock chick talking about chaos. There is probably a crowd that would find that fun.
So back to how to make a level fun? Know your audience.